Bay y 5959 is a positive dihydropyridine inotrope that selectively promotes Ca2+ entry into myocardial cells, probably by a direct, Ca2+ channel mechanism, minimizing the increase in O2 consumption compared to catecholamines or phosphodiesterase inhibitors. The Ca2+ promoter action was studied on rat myocardial and myovascular cells during tight seal pipette voltage clamp. Bay y 5959 (30 nM) markedly slowed Ca2+ channel activation and deactivation in a narrow range of membrane potential between -50 and -30 mV that is apparently unique. No other compound slows and prolongs Ca2+ currents so strongly or for such a long duration. For example, in comparison with Bay K 8644, there was an amplitude increase of about 2-fold with both drugs, but Bay y 5959 prolonged the duration of the current by more than 10 times as much as Bay K 8644. There was also selectivity of Bay y 5959 (but not Bay k 8644) for cardiac over vascular muscle cells. Bay y 5959 had a significant enhancing effect on Ca2+ current in both cardiac and vascular muscle cells, but the cardiac/vascular selectivity ratio was about 5 times, based on integral of Ca2+ current. Vascular muscle amplitude increased by less (only by about 30%) and duration was only about doubled, and thus the total Ca2+ current increase was only about 2.2 rather than about 12 times. These cellular mechanistic data suggest that Bay y 5959 is a useful positive inotrope, with cardiac selectivity, that has a unique pharmacological profile.